1. Field of the Invention
The present invention relates to a process for producing a metal article, the surface of which has been pretreated with a silane composition and then extrusion-coated with a polyamide molding composition. More particularly, the invention relates to correspondingly extrusion-coated metal profiles.
2. Description of the Background
Metal articles intended for applications in which they have exposure to corrosive environments may be extrusion-coated with a polyamide molding composition. This coating not only acts as a barrier to access of a corrosive agent to the metal surface, but also gives the surface some degree of mechanical protection, for example against exposure to friction or impact. This protection is especially desirable if the surface prior to coating has also undergone treatment for corrosion protection. An essential precondition here is that the coating applied by extrusion adheres firmly, so that mechanical damage does not cause the corrosive agent, such as water or aqueous saline solution, to migrate along the underside of the coating.
If the metal article is composed of aluminum or of an aluminum alloy, for example, it has been standard practice to pretreat the metal by chromatizing the metal. The chromatizing firstly improves the adhesion of the polyamide molding composition applied, and even the chromate layer itself brings about an improvement in corrosion resistance. Examples of appropriate applications in the automotive industry include fuel lines, servo control lines and water-cooling lines. This method provides an adequate improvement in corrosion resistance, and also in the adhesion of the polyamide layer. However, the carcinogenic property of the chromium(VI) compounds used in the chromatizing bath is causing the automotive industry to eliminate chromatizing as metal pretreatment step. A need, therefore, exists for a suitable metal pretreatment process which is acceptable as a replacement for chromatizing and also which imparts the metal surface with adequate protection from corrosion and at the same time enables the applied polyamide layer to firmly adhere to the metal. It would also be desirable to be able to use this method to provide effective corrosion protection to articles composed of other metals, such as steel, brass or zinc.
JP-A 9-169079 discloses that an aluminum sheet can be firmly bonded to a polyamide film if the aluminum sheet is first treated with the aqueous solution of an epoxy silane and then, after drying, a polyamide film is used to produce a compression bond at a temperature at which the polyamide is molten. In the examples a bond with nylon-12 is produced by hot lamination for a period of 2 minutes at 240xc2x0 C. These results are not transferable to extrusion coating, since no significant pressure is exerted on the bond here, and especially since the contact time during which the polyamide applied remains molten and can bind to the surface is of the order of magnitude of a few seconds. It could not, therefore, be expected that the suitability of silanes as adhesion promoters will extend to the extrusion-coating of metals with polyamide, giving the desired corrosion protection.
Accordingly, one object of the present invention is to provide metal surfaces with adequate protection from corrosion by the application of an extruded coating thereto which at the same time enables a subsequently applied polyamide layer to firmly adhere to the metal.
Briefly, this object and other objects of the present invention as hereinafter will become more readily apparent can be attained by an extrusion-coated metal article which is prepared by a process comprising
1) applying a mixture which comprises an organosilane composition which is prepared from
a) Q mols. of alkoxysilanes which have a functional group and have the formula:
Axe2x80x94Si(R1)y(OR*)3-y, and xe2x80x83xe2x80x83I 
b) M mols. of alkoxysilanes selected from the group consisting of
xcex1) trialkoxysilanes of the formula:
R2xe2x80x94Si(OR**)3 xe2x80x83xe2x80x83II 
and/or from the group consisting of
xcex2) dialkoxysilanes of the formula:
R3R4Si(OR***)2 xe2x80x83xe2x80x83III 
and/or from the group consisting of
xcex3) alkoxy compounds of the formula:
Me(OR****)n, xe2x80x83xe2x80x83IV 
wherein A is a substituent which has, bonded directly or via an aliphatic or aromatic hydrocarbon radical to silicon, thereto at least one amino, alkylamino, dialkylamino, amido, epoxy, acryloxy, methacryloxy, cyano, isocyanato, ureido, thiocyanato, mercapto, sulfane or halo group,
R1 is methyl, ethyl or A which is defined above,
y is 0 or 1,
R*, R**, R*** and R****, independently of one another, are an alkyl group having from 1 to 8 carbon atoms or a corresponding alkyl group which is substituted by an alkyl[(poly)ethylene glycol] radical,
R2, R3 and R4, independently of one another, are an alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkenyl or aromatic group, in each case having not more than 18 carbon atoms, or a group of this type which is partially fluorinated or perfluorinated and/or is substituted by alkyloxy and/or by aryloxy groups,
Me is selected from the group consisting of Si (n=4) and metals, such as Ti (n=4), Zr (n=4),Al (n=3)and Zn (n=2),
with the proviso that
the molar ratio of M and Q is 0xe2x89xa6M/Qxe2x89xa620, preferably 0xe2x89xa6M/Qxe2x89xa612, particularly preferably 0xe2x89xa6M/Qxe2x89xa67 an very particularly preferably 0xe2x89xa6M/Qxe2x89xa64
on average, at least 2.4 alkoxy groups OR*, OR**, OR*** or, respectively, OR****, particularly preferably at least 2.5 alkoxy groups and very particularly preferably at least 2.6 alkoxy groups, are present in the mixture of the compounds I to IV for each silicon atom and, respectively, metal atom, and
if concomitant use is made of one of the metal compounds mentioned as compound IV, the atomic ratio of metal/Si in the mixture of compounds I to IV is not more than 4:6, preferably not more than 3:7 and particularly preferably not more than 2:8, to a metal surface; and
2) extruding a melt of a polyamide molding composition onto the surface thus treated and dried.